Axial-gap rotating electric machines have advantages in that the thickness thereof can be reduced and a large torque can be obtained, compared with radial-gap rotating electric machines in which a stator is disposed on the outer peripheral side of a rotor. Therefore, application of axial-gap rotating electric machines to electric automobiles and the like is anticipated. An axial-tap rotating electric machine has a structure in which a stator including an excitation coil and a rotor including a permanent magnet are disposed with a very small gap (axial gap) therebetween in the axial direction. The rotor includes a disk-shaped substrate, which functions as a back yoke, and a plurality of permanent magnets disposed and fixed on a surface of the disk-shaped substrate. Typically, each of the permanent magnets has a sectoral shape, and north poles and south poles for a desired pole number are alternately arranged annularly.
The permanent magnet is exposed to a magnetic field that changes with time when the rotor rotates. Therefore, Joule heat is generated in the permanent magnet due to an eddy current, and thereby a problem occurs in that the permanent magnet is heated and the magnetic force of the permanent magnet decreases. In order to reduce the Joule heat, it is effective to reduce the magnitude of the eddy current by dividing the permanent magnet into pieces as described, for example, in PTL 1.
How to fix a plurality of permanent magnets to the disk-shaped substrate of the rotor is a common problem for axial-gap rotating electric machines. That is, because the rotor faces the stator with a very small axial gap therebetween, a method that can be used to fix the permanent magnets is limited. In particular, in the case where the permanent magnet is divided into pieces in order to reduce the eddy current, it is difficult to securely fix the magnetic pieces.
In PTL 1, a method of fixing magnetic pieces to each other by using an adhesive is disclosed. The fixing method using an adhesive is advantageous in that a mechanical fixing member is not used and is also advantageous for joining the substrate and the permanent magnets, which differ from each other in coefficient of linear expansion. However, even when the permanent magnet is divided into pieces, an eddy current is generated in the permanent magnet and generates heat. As the temperature of the permanent magnet increases, generally, the adhesive becomes degraded, and a problem may occur in that the permanent magnet fixed by the adhesive is removed from the substrate.